A metal pigment has conventionally been used for metallic paints, printing ink, plastic kneading and the like for the purpose of obtaining a beautiful decoration effect in which much importance is attached to a metallic feeling. Although an aluminum pigment having a surface not subjected to any treatment has a high metallic feel and designability, it has poor adhesiveness with a resin in a coating film depending on a resin system of coating materials and printing ink. As a result, it had disadvantages of easy peeling when an adhesion test by cellophane tape peeling was performed, and of having no protective function to a chemical.
This is probably because compatibility and wettability between the surface of aluminum pigment and a resin of coating materials and printing ink are insufficient. Therefore, a method of subjecting the aluminum pigment to surface treatment has been proposed as a measure of improvement thereof.
Patent Document 1 proposes a method in which a flaky aluminum powder or a paste of the flaky aluminum powder, which is an aluminum pigment, is dispersed in an organic solvent, first allowing a radically polymerizable unsaturated carboxylic acid or the like to be absorbed by the powder, and then coating the surface of the powder with a polymer produced from a monomer having 3 or more radically polymerizable double bonds. Although this method improves adhesiveness, it has a problem that it is necessary to add a considerable amount of a coating monomer in order to achieve chemical resistance of a metallic coating film that is originally the main purpose, resulting in simultaneous reduction in metallic feel and significant reduction in designability.
Patent Document 2 proposes a method for improving a surface coating method to form a coating film that is uniform and highly three-dimensionally crosslinked in order to prevent reduction in the color tone of a resin-coated metal pigment and to further improve chemical resistance and weatherability. However, although reduction in metallic feel and designability is improved to some extent by this method, the color tone of a resin-coated metal pigment is considerably inferior to the color tone of a metal pigment which has not been subjected to surface coating treatment, and is not sufficient.
Further, in a region where the average particle size is 10 micrometers or less, in which particularly high designability is required recently, the problem is that the average particle size after resin coating treatment is significantly increased compared with that before the treatment, resulting in significant reduction in color tone such as luster, hiding properties, and a flip-flop feeling when a coating film is formed.
Patent Document 3 discloses a method of using an ultrasonic wave when mica is coated with a resin. However, an aluminum pigment is not mentioned, and there is no discussion at all with respect to color tone, chemical resistance and adhesiveness which are important properties for an aluminum pigment.